(1) Field of the Invention
The present invention pertains to the field of nuclear magnetic resonance imaging. More particularly, the present invention pertains to methods of performing segmented echo planar nuclear magnetic resonance imaging.
(2) General Background
Interleaved segmented echo planar imaging (SEPI) (also referred to as multi-shot) offers a fast approach for high resolution volumetric magnetic resonance imaging using a variety of imaging techniques. Interleaved SEPI provides higher image resolution and superior handling of off-resonance behavior as compared to single shot (non-segmented) echo planar imaging.
The echo time shifting (ETS) technique can be used in SEPI imaging methods to effectively smooth the discontinuities between regions of data and create a ghost free magnitude image. However, phase evolution in the phase encoding direction with linear ky dependence present in the ETS technique puts back large geometric distortion similar to that which occurs using single shot echo planar imaging techniques. The application of improved gradients and calibration techniques in combination with echo time shifting techniques effectively eliminates ghosting or blurring resulting from phase discontinuities between segments. However, such prior art techniques fail to eliminate distortion resulting from off-resonance phase shifts in the phase encoding direction, and these undesirable phase shifts remain problematic. This is especially the case when generating high resolution images using alternate echo sampling to achieve better flow properties during imaging related to magnetic resonance angiography. If phase evolution is not accounted for when using a SEPI technique, the merging of k-space lines will yield an image with distortion in the phase encoding direction.
Field mapping methods effectively reduce image distortion originating from static off-resonance effects resulting from such things as Bo field inhomogeneity and chemical shift. The field map is typically derived from phase information obtained from either a double-echo gradient echo or offset spin-echo images. However, the phase residual resulting from the complex division of images having different echo times must be unwrapped to ascertain the field inhomogeneity map.
The use of multiple reference scans and multi-echo gradient-echo imaging reduces both Bo field inhomogeneity and eddy current effects. Moreover, phase shifted echo planar imaging pulse sequences that take into account all off-resonance effects including gradient waveform imperfections have been developed. However, all such methods require appreciable extra time and heavy post processing as compared to methods that do not compensate for off-resonance effects. As such, inter-scan motion may even reduce the practical application of such methods.